The aim of the experiments proposed is to elucidate the mechanism by which phosphorylating conditions activate tyrosine hydroxylase, a key and rate-limiting enzyme in the biosynthesis of catecholamines. This activation requires a cAMP-dependent protein kinase but apparently does not result in the direct phosphorylation of catalytic tyrosine hydroxylase. Highly purified enzyme systems will be used to determine: (a) the optimal components of the phosphorylated moiety, (c) the manner in which this moiety interacts with tyrosine hydroxylase if the enzyme itself is not phosphorylated, and (d) an estimation of the physiological role such an activation might effect. It is also proposed to develop a procedure for the isolation of pure tyrosine hydroxylase in high yield. This should make possible the isolation of tyrosine hydroxylase from tissues in which this enzyme plays an important role but is present only in small amounts e.g., vasculature or peripheral symphathetic ganglia.